US3097481A

US3097481A - Silver

Info

Publication number: US3097481A
Authority: US
Publication date: 1963-07-16
Anticipated expiration: 1980-07-16

Description

July 16, 1963 Filed Nov. 24, 1959 B. SILVER PROPELLENT GRAINS 3 Sheets-Sheet l I N VENTOR ezmfd J'JZVer BY d0 July 16, 1963 B. SILVER PROPELLENT GRAINS 3 Sheets-Sheet 2 Filed Nov. 24, 1959 I Il Il lfl Il II Il I f lu/'d @Yayi/f BY @w July 16, 1963 B. sxLvER PROPELLENT GRAINs 3 Sheets-Sheet 5 Filed Nov. 24, 1959 AGE/V7 23,09'13481 PROPLLENT GRAINS Bernard Silver, Alexandria, Va., assignor to Atlantic .Research Corporation; Fairfax County, Va., a corporation of V' inra ll'gFilerl Nov. 24, 1959, Ser. No. 855,240

9 Claims. (Cl. Gli-35.6)

This invention relates to new and improved propellent grains, the mass burning rate of which can be controllably varied during burning, with concomitant controlled variation of mass rate of 'gas generation and thrust.

It is well-known that one of the factors determining situation exists in the case of to give substantially constant cult nozzle controls,V

given gasgener also necessary to com of the propellant at thepoiiit tion in performance for the ambient temperature Unscheduled variaas wires, f of the initial ignition surly disposed in the direction .it Y in, have recently been in- Such -wj'red grains have eliminated Y wiel-burning grains by greatly the elective niifieportion of propellent grain the rnetalwstlV gaat the propellent matrix 3,097,481 Patented July 16, 1963 tageous since they reduce grain strength and motor loading capacity.

Wired end-burning grains, like conventional end-burnrOther objects and advantages will become obvious from the following detailed description and the drawings:

In the drawings, in which like parts in the several ligures are identilied by the same reference characters:

FIGURE l is a longitudinal sectional view through a rocket motor showing invention.

FIGURE 2 FIGURE 1.

FIGURE 3 is an enlarged detail view of the ture regulating means shown in FIGURE I.

FIGURE 4 comprises a diagrammatic series of longitudinal sectional views showing the effect on mass burning rate of metal heat conductor tubes at different temperatures.

FIGURE 5 is tion.

FIGURE 6 is URE 5.

FIGURE 7 is a fragmentary longitudinal section of still another modification.

FIGURE 8 is a cross section taken on 8-8 of FIG- URE 7.

is a cross-sectional view taken on 2 2 of temperaa longitudinal section showing a modificaa cross section taken on lines 6 6 of FIG- the ambient temperature of the propellent matrix. It is a well-known characteristic of propellants in general that the linear burning rate increases with increased ambient temperature of the matrix and decreases with decreasing ambient temperature.

The metal tubes of my invention function, as do wires or metal heat conductors of any other shape. to produce involution of the burning surface of the grain along the tubes and, thereby, an increased burning surface area. The passage of heating or cooling fluids through the tubes serves as a modulating means for varying the burning rate of the grain along the metal conductor.

Heating the tubes by means of the uid to a temperature above the ambient or environmental temperature of the grain matrix increases the burning rate of the matrix along the tube. Cooling reduces the burning rate. It should be noted, however, that regardless of the temperature change induced in the embedded metal tubes by passage of the Huid therethrough, the tubes continue to function as heat conductors from the hightemperature combustion zone into the propellent matrix to produce involution of the burning surface and mass rate of gas generation. It is the degree of such involution and, consequently the total burning surface area, which is controllably varied by changing the temperature of the metal tubes.

The mechanism by which variation in mass burning rate is controllably produced by controllably varying the temperature of the embedded metal conductor has not been definitely established. It appears probable, however, that heating or cooling the conductor relative to the ambient temperature of the propellent matrix effects a similar change in the layer of propellent matrix adjacent to and in intimate contact with the metal tube. This change in temperature changes the linear burning rate of this portion of the matrix, increasing the rate with an increase in temperature and reducing the rate with a decrease in temperature. Where the layer of matrix adjacent to the metal conductor has a higher burning rate because of an increase in its temperature, the burning rate along the wire increases, with accompanying increased degree of involution, burning surface area, mass rate of gas generation, and thrust. Conversely, a decrease in the temperature of the layer of matrix in contact with the metal conductor, decreases the burning rate of this portion of the matrix and decreases the extent of involution of the burning surface along the conductor.

Thus, by providing means for lcontrollably passing fluids at higher 0r lower temperature-s at controlled rates through the heat conductor tubes during the burning cycle, the mass burning rate ot' the propellent grain can be controllably modulated at will to compensate for conditions met during operation, as for example, during flight of a rocket.

As aforementioned, the tiuid employed to modulate the temperature can be a liquid or a gas. In general, a liquid is the preferred medium for effecting an increase in temperature of the metal tube conductor, and a gas is preferred for cooling. The fluids are most conveniently stored in pressurized storage tanks forward of the combustion chamber containing the propellent grain, whence they are controllably fed by means of a Suitable valve system through the metal tubes longitudinally embedded in the grain from the forward end of the grain to the combustion zone adjacent to the rearward end of the combustion chamber, eg., adjacent to the nozzle of a rocket motor.

Since the fluids are employed primarily as a means for heating or cooling, substantially any Huid can be employed for my purpose and, in general, will be chosen for compatibility, temperature properties, eg., boiling point, and ease of handling in a given application. The choice of a liquid, for example, will be, to a considerable extent, determined by the particular temperature requirements for `a given propellent system. Where relatively high heating temperatures axe likely to be required, a high boiling point liquid is preferred, such as a paraffin oil, eg., kerosene', a polyglycol, eg., diethylene glycol', esters, eg., dibutyl phthalate, dioctyl sebacate; dibutyl adipate, etc. Lower boiling point liquids, such as water and alcohols, e.g., ethanol, isopropanol, etc., can be used. The high pressure conditions under which the liquids are ernployed, since they must be moved through the tubes against the high pressure combustion gases in the combustion chamber, operate to increase the effective boiling point of the liquid heating medium.

The liquids can be heated to the desired temperature for use as a heating medium in any convenient manner, as for example, by means of `heating elements in or surrounding the tanks in which they are stored forward of the propellent grain, or by filling well-insulated tanks with the heated liquid shortly prior to operation or take-off.

Temperature control by means of the heated uid medium can be obtained both by regulating its temperature and by varying its rate of flow through the metal tubes, the latter expedient generally being the more practically convenient. rThus the tiuid can be maintained in its storage chamber at a certain maximum temperature, and the degree of heat transfer metal conductor regulated by the rate at which the heated huid is passed through the metal tubes. Since internal ignition of the end-burning grain must be avoided, the maximum temperature of the heating tiuid during passage through the metal tubes should be such as not to raise the temperature of the metal conductor to the auto-ignition of the propellant. This temperature will, of course, vary with the particular ignition characteristics of the propcllent compositions in which the tubes are embedded.

Since a gas cools upon expansion, the use of `a compressed gas provides an excellent and convenient coolant means. A suitable gas can be maintained in a storage tank forward of the combustion chamber at a pressure substantially higher than the pressure off the combustion gases in the combustion chamber and can be controllably fed into the metal heat conductor tubes at a rate controlling the degree of gas expansion and, thereby, the degree of cooling. Any convenient gaseous medium can be employed for the purpose, such as air, oxygen, nitrogen, and the like. In some instances the fluid can be in liquid state under the high pressure in the storage tank and vaporize into a gas upon expansion into the metal heat conductor tubes.

The tluid, after passage through the metal tubes, vents into the combustion zone, where it mingles with the gaseous combustion products of the propellant `and then vents out the nozzle of the motor. Liquids, of course, vaporize at the high temperature of the combustion zone. The temperature-controlling fluids will normally be employed in such relatively small amounts as not appreciably to affect performance by virtue of their dilution of the combustion gases.

The metal tubes can shape, such as circular, oval, or `drical tube is, in general, preferred stress-resisting characteristics.

The tubes `are preferably made of silver, copper, or aluminum, and can be fashioned from any other metal or metal alloy having good heat conductive properties, such as platinum, tungsten, magnesium, molybdenum, steel, and the like. To a considerable extent, the particular metal used will be determined by the mass burning rate and stress-resisting requirements for a given application.

As aforementioned, the tubing must be in intimate, gassealing contact with the propellent matrix along its entire length within the grain. This intimate contact is essential to effectuate control of the burning rate of the matrix by means of the embedded metal conductor. Any spacing of the metal heat conductor from the matrix results only in the establishment of an exposed surface in the interior of the grain which ignites and then burns progressively be of any suitable cross-sectional rectangular. A cylinbecause of its superior Y oxidant mum internal diameter being about 0.1 50 inch.

Before the llame actively propagates along the metal heat conductor tube, a short length of the metal must pro- ."shorter 1s the len of exposed conductor required be- For effective action, therefore, of sufficient length both to provide for the initial exposure into the llame zone and for propagation of the ame for some distance into the unburned propellant in which it is embedded. In general minimum length of conductor required to achieve appreciable increase in to 0.1 inch and, preferably, about 0.2 inch.

' The propellent matrix can be any suitable self-oxidant composition which,

burns to produce pro- H2 and H2O. By selfwhich contains within itpulsive gases, such as CO, C02,

is meant a composition glycerme, or of the composite tu] organic fuel and a finely-divided inorganic solid oxidzer.

The matrix can be a conventional solid propellant or a plastic semi-solid. L Cohesive, shape-retcntive monopropellent compositions Ywhich are characterized as plastic er semi-solid because they peratures under moderate stress or pressure, can be loaded into the combustion chamber of a ga -generating device continuous matrix The physical propin terms of shapeand thixotropy, can agent or by using a viscosity, such as a An example of a semi-solid monopnopellent composition suitable for use as an end-burning grain is one consisting taf-79.7 0 NH4ClO4, 12.1% dioctyladjpate, 8.1% polyvinyl chloride (gelling agent), and 0.1% wetting agent, the precentages being by weight.

111 FIGURES l, 2, and 3 of the drawings, for illustrative purposes, propellent grain 1 is shown in the combustioachamber 2 of rocket motor 3, equipped with restricted nozzle 4. The end-burning grain is inhibited on its lateral surfaces by inhibitor coating S and on its forward end oi an oxidizable orga-nic liquid fuel.

orties of the plastic mfonopropellant, retentive oohesivene s, tensile strength hemproved by addtion'of a gelling liquid vehicle of substantial intrinsic liquid organic polymer.

by plastic cement 6 bonding the grain to forward wall 24 of the combustion chamber. Longitudinally embedded in the grain, normal to initial ignition surface 7, is metal heat conductor tube 8, in intimate, gas-Sealing Contact with the propelient matrix, and opening rearwardly into the rearward portion 9 of the combustion chamber adjacent to the nozzle.

Forward of the propellant grain and the combustion chamber is storage tank 10 containing a gas 23, Such as air or oxygen under pressure which is higher than combustion chamber pressure during the burning cycle of the grain. The storage tank communicates through channel 11 with manifold 12, which, in turn, communicates through channel 25 with embedded metal tube 8 through oritice 26 in wall 24. Passage of the gas into tube 8 is controlled by needle valve 13, which is either closed as to the desired degree by conventional, remotely controlled valve actuator means 14.

Storage tank 15 contains liquid 16, such as water or dipressurized by compressed of the grain. Electrical resistance element 19 in the wall of the tank, powered by battery 27, keeps the liquid at the desired high temperature. Passage of the liquid through channel 20 into manifold 12 and thence into metal tube 8 is controlled by needle valve 21, which is either completely closed as shown or opened to the desired degree by remotely-operated valve actuator means 22.

To avoid internal heating of tube 8 by the combustion gases produced upon ignition of the end-burning surface 7, the tube and manifold I2 can be lled with fluid 28, preferably a liquid, as shown, though it can also be a gas, at any time after assembly of the motor. This can be accomplished, for example, by opening valve Z1 (or valve 13) sutlciently to fill the tube and manifold and then closing it. Closure disc 29, preferably made of plastic, such resin, which is rupturable under pressure er volatilizes after ignition of the grain, seals the liquid in tube 8 until ignition. tlf such liquid is at the ambient temperature of tube 8 and the propellant grain matrix, burning rate along the tube is not affected by its presence. If, after ignition, valves 13 and 21 are kept closed, the initially introduced liquid will remain within the tube under the compression produced by the combustion gases in zone 9.

In the embodiment illustrated in FIGURE l, tube 8 can either be heated by the passage of hot liquid from controlled by the valve, or cooled fluid will be sullcient.

FIGURE 4 illustrates diagrammatically the equilibrium burning surfaces obtained with the same grain, but with different conditions prevailing in the metal heat conductor tube. After ignition of the propellant grain, in

tube to form cones a, b, and c, with the with liquid `28 at ambient temperature of the grain, introduced prior to ignition. metal tube is thus a short end 3i)` protruding above the burning surface.

In FIGURE 4B, heated through tube 8b and vents into the combustion zone where it vaporizes. The metal Walls of the tube are heated above the ambient temperature of the propellent matrix; the burning rate along the metal heat conductor tube increases; and a more acute, deeper cone b, of larger burning surface area than cone a, is formed. In FIG- URE 4c, gas 23 is expanded into the tube from a storage tank of compressed gas, and after passing through the tube, expands into the combustion zone 9. The metal heat conductor tube is thus cooled below the ambient temperature of the propellent grain matrix, producing a reduced burning rate, a more obtusely angled cone c, and a smaller burning surface area.

In most cases, and, particularly Where the propellent grain has a relatively large cross-sectional area, it is desirable to embed a plurality of the metal heat conductor tubes 8 longitudinally at spaced intervals, as shown in FIGURE 5. If a propellent grain contains only a single metal heat conductor, as shown in FIGURE l, the peripheral portion of unburned propellant remaining when burning has progressed the full length of the metal tube may be larger than is desirable. This can be avoided by introducing a plurality of conductors. FIGURE 5 illustrates an application in which only a heating liquid, stored in tank 15, is employed. The manifold 12 is provided with a plurality of channels 25 in registry with orifices 26 in the rear motor wall 24 and with tubes 8. 'Ihe spacing of the tubes relative to each other is not critical and is determined by the particular requirements of a given application, the spacing being such as to provide for the desired degree of burning surface involution.

It is also frequently desirable to achieve or approach the equilibrium burning surface, namely the maximum involution produced by the metal heat conductor under particular operating conditions, as quickly as possible. The use of a plurality of conductors, as shown in FIG- URE 5, greatly increases the rapidity with which this can be accomplished, since the involutions incident to the metal conductors soon intersect at their flaring ends. Al-

though the apex angle of the recess for each of a plurality of conductors is the same for a single conductor of the same size and shape embedded in a grain, the depth of the recessed cones is shorter in the case of a plurality of conductors, so that overall burning surface is not in actuality increased.

The equilibrium burning state can also be approached more rapidly by exposure of the metal heat conductor tubes a short distance beyond the initial ignition surface as shown at 32 in FIGURE 5, or by prerecessing the initial ignition surface with the end of the tube exposed at the apex of the recess, as illustrated by coned recesses 33 in initial ignition surface 34 in FIGURES 7 and 8.

As aforedescribed, upon ignition, the grain burns for a short distance at the normal rate of the propellent material itself until a short length of the metal protrudes into the hot combustion gas zone, before the llame propagates along the metal heat conductor tube. Initial protrusion of the conductor, therefore, more quickly initiates the desired rapid ame propagation.

Prerecessing of the ignition surface also eliminates at least :a portion of the progressivity produced when the burning surface regenerates from an initial plane ignition surface to its maximum involuted state along the metal tube.

The embedded metal heat conductor tubes make possible increases in effective burning rate which are as much as 3 to 5 times larger than that of the propellent matrix itself. Variation in elective or mass burning rate and, thereby, thrust modulation can be varied in a given propellent grain within the range determined by the maximum rate obtainable by heating the tubes to the maximum practical degree by passage therethrough of heated Huid, and the minimum rate obtainable by cooling the tubes, such variation being accomplished at will during the burning cycle of the grain by controlled passage of the heating or cooling fluids through the tubes. It will be understood that controlled passage of the temperature-controlling fluids can be maintained at controlled rates throughout the burning cycle of the grain or only during a portion of the burning cycle at any point or points in the cycle when conditions require a change in the mass rate of gas generation.

It is recognized that the system of my invention involves an increase in dead load. This, however, can generally be maintained within practical limits, especially where the modulated propellent grain is relatively large, since the amounts of fluid normally required are small, particularly Where tubes of minimum internal diameter and wall thickness are employed, and since the lluid storage chambers and the actuating and controlling means can be maintained outside the combustion chamber and, therefore, outside the sphere of the high temperature, corrosive combustion gases, thereby making possible the use of strong but light-weight structural materials.

Although the invention has largely been described in terms of rocket motor application, it can effectively be used in any gas-generating device employing a propellent grain as a source of propulsive gases, as, for example, in catapult launchers or turbines. In such applications, it should be noted, the weight of the temperature-modulating means is of no practical consequence.

I claim:

l. A propellent grain, said grain being designed to burn continuously from one end which is an initial ignition surface and comprising a self-oxidant propellent matrix, the combustion of which generates propellent gases. said matrix containing embedded therein an elongated metal heat conductor tube positioned substan- V tially normal to the plane of said initial ignition surface of said grain and extending continuously in the direction of flame propagation for the entire length of said grain, the entire exterior surface of Said metal tube lying within the body of the propellent grain being in intimate, gas-sealing contact with the propellent matrix, the maximum wall thickness of the said metal tube being about 0.05 inch and the maximum internal diameter of the tube being about 0.6 inch, said metal heat conductor tube being adapted to serve as a channel for controllable passage therethrough in contact with the interior wall surface of said metal tube, during the burning cycle of the grain, of fluid, which vents out of the burning end of said grain, said uid being at a temperature different from and, thereby, changing that of said tube.

2. A propellent grain, said grain being designed to burn continuously from one end which is an initial ignition surface and comprising a self-oxidant propellent matrix, the combustion of which generates propellent gases, said matrix containing embedded therein an elongated metal heat conductor tube positioned substantially normal to the plane of said initial ignition surface of said grain and extending continuously in the direction of flame propagation for the entire length of said grain, the entire exterior surface of said metal tube lying within the body of the propellent grain being in intimate, gas-sealing contact with the propellent matrix the maximum wall thickness of the said metal tube being about 0.015 inch and the maximum internal diameter of the tube being about 0.150 inch, said metal heat conductor tube serving as a channel for controllable passage therethrough in contact with the interior wall surface of said metal tube, during the burning cycle of the grain, of uid, which vents out of the burning end of said grain, said uid being at a temperature different from and, thereby, changing that of said metal tube.

3. The propellent grain of claim 2 containing a plurality of said elongated metal heat conductor tubes in spaced relationship each to the other.

4. The propellent grain of claim 3 in which one end of each of said metal heat conductor tubes is exposed at the apex of a recess in said initial ignition surface.

5. A gas generator device comprising a combustion chamber, a propellent grain seated therein, said grain being designed to burn continuously from one end which is an initial ignition surface and comprising a self-oxidant propellent matrix, the combustion of which generates propellent gases, said matrix containing embedded therein an elongated metal heat conductor tube positioned substantially normal to the plane of said initial ignition surface of said grain and extending continuously in the direction of ame propagation for the entire length of said grain, the entire exterior surface of said metal tube lying within the body of the propellcnt grain boing in intimate, gas-sealing contact with the propellent matrix the maximum wall thickness of the said metal tube being about 0.05 inch and the maximum internal diameter of the tube being about 0.6 inch, said metal heat conductor tube serving as a channel for controllable passage therethrough in contact with the interior wall surface of said metal tube, during the burning cycle of the grain, of fluid which vents out of the burning end of said grain, and means, positioned outside of said combustion chamber, for providing said fluid at a temperature below the au-toignition temperature of said propellent grain and different from that of said metal tube embedded in said propellant grain, and for passing said tluid through said metal tube at a controllable rate.

6. The gas generator device of claim 5 in which the propellent grain contains a plurality of said elongated metal heat conductor tubes in spaced relationship each to the other.

7. The gas generator device of claim 6 in Which the fluid-providing means comprises a storage chamber containing gaseous iluid under higher pressure than the pressure in said combustion chamber during the burning cycle of said propellent grain, communicating means between said storage chamber and said metal heat conductor tubes in said grain, and controllable valve means capable of controlling flow of said gaseous iluid into said metal heat conductor tubes, said gaseous uid, when owing through said metal heat conductor tubes serving as a coolant for said tubes.

8. The gas generator device of claim 6 in which the uid-providing means comprises a storage chamber containing a liquid at a higher temperature than that of the propellent grain and the metal heat conductor tubes em bedded therein, communicating means between said storage chamber and said metal heat conductor tubes, pressurizing means for forcing passage of said liquid from said storage chamber through said metal heat conductor tubes, and controllable valve means capable of controlling flow of said liquid into said metal heat conductor tubes.

9. A gas generator device comprising a combustion chamber, a propellent grain seated therein, said grain being designed to burn progressively from one end which is an initial ignition surface and comprising a self-oxidant propellent matrix, the combustion of which generates propellent gases, said matrix containing embedded therein an elongated metal heat conductor tube positioned substantially normal to the plane of said initial ignition surface of said grain and extending continuously in the direction of flame propagation for the entire length of said grain, the entire exterior surface of said metal tube lying within the body of the propellcnt grain being in intimate, gas-sealing contact with the propellent matrix, the maximum Wall thickness of the said metal tube being about 0.05 inch and the maximum internal diameter of the tube being about 0.6 inch, said metal heat conductor tube serving as a channel for controllable passage therethrough in contact with the interior wall surface of said metal tube, during the burning cycle of the grain, of fluid which vents out of the burning end of said grain, and means, positioned outside of said combustion cham- Vber, for providing said fluid at a temperature below the autoignition temperature of said propellent grain and different from that of said metal tube embedded in said propellent grain, and for passing said iluid through said metal tube at a controllable rate.

References Cited in the file of this patent UNITED STATES PATENTS 2,419,866 Wilson Apr. 29, 1947 2,618,120 Papini Nov. i8, 1952. 2,816,721 Taylor Dec. 17, 1957 2,958,183 Singelmann Nov. l, 1960

Claims

1. A PROPELLENT GRAIN, SAID GRAIN BEING DESIGNED TO BURN CONTINUOUSLY FROM ONE END WHICH IS AN INITIAL IGNITION SURFACE AND COMPRISING A SELF-OXIDANT PROPELLENT MATRIX, THE COMBUSTION OF WHICH GENERATED PROPELLENT GASES, SAID MATRIX CONTAINING EMBEDDED THEREIN AN ELONGATED METAL HEAT CONDUCTOR TUBE POSITIONED SUBSTANTIALLY NORMAL TO THE PLANE OF SAID INITIAL IGNITION SURFACE OF SAID GRAIN AND EXTENDING CONTINUOUSLY IN THE DIRECTION OF FLAME PROPAGATION FOR THE ENTIRE LENGTH OF SAID GRAIN, THE ENTIRE EXTERIOR SURFACE OF SAID METAL TUBE LYING WITHIN THE BODY OF THE PROPELLENT GRAIN BEING IN INTIMATE, GAS-SEALING CONTACT WITH THE PROPELLENT MATRIX, THE MAXIMUM WALL THICKNESS OF THE SAID METAL TUBE BEING ABOUT 0.05 INCH AND THE MAXIMUM INTERNAL DIAMETER OF THE TUBE BEING ABOUT 0.6 INCH, SAID METAL HEAT CONDUCTOR TUBE BEING ADAPTED TO SERVE AS A CHENNEL FOR CONTROLLABLE PASSAGE THERETHROUGH IN CONTACT WITH THE INTERIOR WALL SURFACE OF SAID METAL TUBE, DURING THE BURNING CYCLE OF THE GRAIN, OF FLUID, WHICH VENTS OUT OF THE BURNING END OF SAID GRAIN, SAID FLUID BEING AT A TEMPERATURE DIFFERENT FROM AND, THEREBY, CHANGING THAT OF SAID TUBE.